The present invention relates generally to tube drawing and more particularly, it relates to plug rolling mills for drawing seamless tubes and taking them off a plug after drawing over the long movable plug.
As compared with other types of tube drawing operations, tube drawing on long movable plugs has an important advantage, namely, the possibility of attaining considerable tube reduction per pass and drawing tubes (with wall reduction) of small diameters (down to capillary ones).
A main disadvantage of this method of drawing tubes on a long movable plug, restricting its application, is a need for labor-consuming additional technological operations, such as: plugging the drawn tube over the plug for withdrawing the latter out of the tube and the plug extraction process as such.
Mills for plugging tubes and taking them off a long movable plug after drawing are known in the art, comprising a plug head adapted to plug the tubes over the plug with the aid of idle tube-reducing members, a pulling device with guides for moving pulling elements therealong, the elements drawing the tube with the plug through the plug head, a gear for taking the tubes off the plug, two pockets wherein the tubes and plugs are placed after the tubes have been taken off the plugs and a drive for the pulling device.
The known designs of the mills aimed at further improvement of the long-lug drawing process as regards tube plugging and taking them off the plugs comprise, as a rule, apart from a drawing line, two additional working lines parallel with one another, namely, a tube plugging line and another line for taking the tubes off the plug.
Mills comprising two pulling devices arranged in line are also applicable, with one of the pulling devices being adapted in this case for pulling the tube together with the plug through a plug arrangement and the other one for extracting the plug from the tube. The time needed for returning the plug (after the tube has been taken off) to the drawing mill is an important factor governing the production rate of the plugging and drawing mills. When using two additional working lines or a single working line with two pulling devices, the flow sheets related to plugging and taking the tubes off the plug envisage periodical stoppage of the mills, reversal of the tube travelling with the plug or turning the plug with the tube through 180.degree..
The above additional operations diminish materially the production of both the plugging and drawing mills.
On the known mills for extracting the plugs from the tubes use is made of thrust plates with a calibrated opening, forks or clamps of the pulling devices gripping the tube fore part on one side and the plug end on the other.
The above operations involved in the extraction of the plugs from the tubes are responsible for the appearance of various defects on the tubes (scratches, galling, corrugation) and for the sticking of tube metal to the plugs.
The arrangement of equipment for plugging and taking the tubes off the plugs when using two additional lines or a single additional working line with two pulling devices requires considerable floor area. For further general details of a typical -- plug rolling mill -- see: The Making, Shaping and Treating of Steel, Chapter XXIX, Manufacturing of Steel - Tubular Products, pp. 1150-1152; a complete revision based on the original text by J. M. Camp and C. B. Francis, Sixth Ed., 2nd Impression, published by United States Steel Company, Copyright 1951 by U.S. Steel Company.